Apparatus for the measurement of rms values

ABSTRACT

There is disclosed an apparatus including the ensemble of a squaring means and an averaging means for the continuous measurement of RMS values which is especially useful for acoustical applications. A storage means in an averager is discharged by constant current which results in a linear discharge function of the averager so that quickly following input signals are no longer covered by the exponential discharge characteristic of the prior art detectors of this kind. The input signal is directly supplied to the one input of a multiplier and is supplied to the other input of said multiplier via an operational amplifier which has another multiplier in its feedback in order to increase the dynamic range of the circuitry.

o J h h United States ate 1 5. [1 1 [11]. 3,7953% Olime et al. 5, T974[54] APPARATUS FOR THE MEASUREMENT 3,714,570 l/l973 Howell 328/26 X RMSVALUES Inventors: Wolfgang Ohme, Boblingen; Primary Exam[ner ]ohn Grund,Sindelfingen; Jorg Winkle? Attorney, Agent, or Firm-A. C. SmithSchonaich, all of Germany [73] Assignee: Hewlett-Packard Gmbl-l,Boblingen,

Germany [57] ABSTRACT [22] plied: July 1972 There is disclosed anapparatus including the ensem- [21] Appl. No.: 270,115 ble of a squaringmeans and an averaging means for the continuous measurement of RMSvalues which is especially useful for acoustical applications. A storage[30] Forelgn Apphcauon Pnonty Data means in an averager is discharged byconstant cur- July Germany P 37 281-6 rent which results in a lineardischarge function of the averager so that quickly following inputsignals are no [52] US. Cl 328/144, 307/230, 307/261, longer covered bythe exponential dissharge charac 1 324/132 teristic of the prior artdetectors of this kind. The [51] Int. Cl. H03k 5/00, GOlv 15/00 inputSignal is directly Supplied to the one input of a [58] held of Search328/26 144; 324/132; multiplier and is supplied to the other input ofsaid 307/230 261 multiplier via an operational amplifier which hasanother multiplier in its feedback in order to increase [56] ReferencesC'ted the dynamic range of the circuitry.

UNITED STATES PATENTS 3,555,432 I/ 1971 Ellermeyer 328/26 5 Claims, 4Drawing Figures Cl II II ATENTEU 51974 3.795.868

sum 1 or 3 FIG E PATENTEDMAR W 3,795,868

SHEET 3 OF 3 FIG 30 APPARATUS FOR THE MEASUREMENT OF RMS VALUESBACKGROUND OF THE INVENTION German patent application Pl,935,544discloses a RMS measuring apparatus where the input signal is suppliedto the two inputs of a multiplier which is connected with an integratingamplifier having its output signal fed back to the multiplier via aproportional feedback connection. By the direct supply of the inputsignal to the connected inputs of the multiplier the range of amplitudeson which the circuitry can operate is limited to half the dynamic rangeof the output circuitry of the multiplier when measuring in logarithmicterms.

In the essay Hochfrequenztechnik und Elektroakustik," I. Sonntag, Volume70, 1961, issue 3, pages 75 to 79 there is disclosed another RMSmeasuring apparatus which is supplied with biased diodes havingdifferent breaking points.

US. Pat. No. 2,839,244 discloses a multiplying means having two inputsfor the signals to be multiplied as well as a divider input.

Furthermore, the squaring means can be constituted of passive or activeelements. This is also true for the averager which in its simplestversion can comprise an RC combination.

The dissertation essay Lautstarkemessgerat fur breitbandige undimpulshaltige Schalle," Pfeiffer, TH Stuttgart, 1966 disclosesmathematical expressions for the response curve 'of a standard circuitrywhen it is supplied with squarewave input impulses, provided that thesquaring means has an ideal characteristic. In this case the leadingedge follows the term and the trailing edge becomes Thus, the chargingaction is essentially shorter than the discharge action, so that smallinput signals which immediately follow substantial preceding signals arecovered and not indicated by the detector. This disadvantage cannot beeliminated by making the time constant 1' of the detector sufficientlysmall. The reason is that in this case signals with a constant RMS valuebut with substantial variations as to the amplitude (for example noise)would result in a variable output instead of the desired constantoutput.

SUMMARY OF THE INVENTION The present invention substantially avoids thementioned disadvantages and provides an RMS measuring apparatus whichhas improved dynamic properties but still is of simple design.

The novel apparatus for the continuous measurement of approximate RMSvalues includes a squaring means and averager having an exponentialtransient characteristic. This averager is connected to said squaringmeans and has a feedback to the squaring means which feedback effects adivision by the value of the output signal of the averager. The averageris discharged by means of a constant current source if and as long asthe measured average value is greater than the value corresponding tothe instantaneous input signal, where the amplitude of the constantcurrent is proportional to the voltage of the averager upon the start ofdischarge action. By such a discharge of the averager it is achievedthat its voltage decreases linearly and that the total discharge actionis always terminated after an unvarying predetermined time interval haselapsed. This discharge action is independent from the level of thevoltage at the time the discharge action is initiated. If the timeconstant of the discharge action is made to be equal to the dischargetime constant 27 of known circuitries the discharge action will now havethe form of a tangential to the (no longer existing) exponentialfunction so that there will be no substantial differences compared withthe prior art systems as long as the signals have only slightvariations.

According to a preferred embodiment of the invention the squaring meansis constituted by a device having two multipliers and an operationalamplifier. In this case the input signal is directly fed to the oneinput of a first multiplier and is fed via the operational amplifier tothe other input of said multiplier. A second multiplier is provided in afeedback loop of the operational amplifier. The amplification and thusthe signal at the output of the first amplifier is inverselyproportional to a signal at the input of the second multiplier. Thisfree input of the second multiplier serves as divider input and isconnected with the output of the averager.

This circuitry has the additional advantage that the first multiplierserving as a squaring means is supplied with a signal which is alreadydivided by the average value so that the voltage level of the signals atthe input is decreased. This circuitry is generally of advantage in RMSmeasuring devices.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of a circuit formeasuring RMS values according to the invention;

FIG. 2 is a diagram of a preferred circuitry constituting a multiplierhaving an increased dynamic range;

FIG. 3a is a schematic diagram of a voltage signal to be measured, forexample the noise generated by the strokes of a typewriter; and

FIG. 3b illustrates in dotted lines the obscured indica tion of a knownRMS measuring apparatus, while the continuous lines illustrate theimproved display achieved by the circuitry according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to FIG. 1 the inputsignal is supplied to an operational amplifier Al via a summationresistor R1. The operational amplifier Al is provided with diodes D1 andD2 as well as with a feedback resistor R2 whereby a half wave rectifierwith an ideal characteristic is constituted. Between the output of therectifier, i.e., between the feedback resistor R2 and the feedback diodeD1 there is connected an input resistor R3 of an integrating amplifierA2. This integrating amplifier is provided with a resistor R4 and anintegrating capacitor C1 and these elements constitute an averager. .Theoutput of the integrating amplifier A2 is supplied to the operationalamplifier Al via an operational amplifier A3 and a summation resistorR5. The operational amplifier A3 is provided with feedback and inputresistors R6 and R7, respectively. The circuitry thus described issimilar to the prior art circuits.

A comparator A4 is connected with the output of the idealized diode viaa resistor R8. The output of the comparator A4 is supplied to the gateelectrode of a field effect transistor TI via a diode D3. The transistorT1 is connected to the output of the amplifier A2 and to a capacitor C2.The output of the integrating amplifier A2 is connected with the gateelectrode of the field effect transistor via a resistor R9. Thecapacitor C2 is connected to the non-inverting input of an amplifier A5which constitutes an emitter follower. The output of this amplifier isconnected with the input R4 of the averager.

This circuitry operates in the following manner:

The input ac voltage is supplied to the amplifier A1 via the summationresistor R1. The amplifier Al operates as a half wave rectifier which inthis embodiment approaches a square characteristic by a conversion curvehaving only one breaking point. However, it will become obvious that thedesired parabolic curve shape can be approached more exactly, forexample by using a squaring circuitry according to FIG. 2 instead of thehalf wave rectifier of FIG. 1. The rectified signal from the idealizeddiode is supplied to the integrating amplifier A2 and will charge theintegrating capacitor C1. During this charging action the samplingtransistor T1 is conductive so that the voltage at the capacitor C2will, without any essential delay, follow the instantaneous value of thevoltage of the capacitor C1. Thus, the resistor R4 will act as if itwere parallel to the capacitor Cl, i.e., it will support the desiredexponential charge action. The output voltage of the integratingamplifier A2 is fed back to the summation input of the idealized diodevia the inverting amplifier A3. Thereby the characteristic curve of theidealized diode and its breaking point are shifted. This shifting willin a known per se manner provide a division of the squared signal by theoutput voltage of the integrating amplifier.

As soon as the diode D1 will become non-conductive and thus point b willbecome more negative than the reference potential which is adjusted atthe resistor R10, the comparator A4 will switch and will block the fieldeffect transistor T1 so that the'capacitor C2 will store the outputvoltage which is present at the integrator at the switching moment. Theconstant discharge current being defined by the voltage of the capacitorC2 and the value of the feedback resistor R4 will effect a lineardischarge of the voltage of the integrating capacitor Cl, until thefield effect transistor will be activated again by a positive voltage atthe output of the idealized diode. At this moment the instantaneousvalue ofthe voltage ofthe capacitor C1 will be supplied to the capacitorC2.

Consequently each input amplitude within the linear range of theelements will be measured within a constant time interval while with theprior art circuitries for measuring RMS value the discharge action ofthe averager was only performed with about 4.3 db per time constant. Thedischarge time is for example 2r, while the charge action will stillfollow the formula provided there are supplied square wave impulses tothe circuitry.

According to the diagram of FIG. 2 the idealized diode between thejunctions a, h, 0 can be replaced by a circuitry including twomultipliers. This squaring means comprises a first multiplier Ml whichserves as the proper squaring means. The input signal is directly fed tothe one input of said multiplier M1 and is fed to the other input ofsaid multiplier via an operational amplifier Al having a controllablegain. The gain can be controlled by the second multiplier M2 in thefeedback loop of the operational amplifier in connection with theresistor R1 and R2 by supplying a dc voltage to the free input of thesecond multiplier M2. There is a reciprocal relation between said dcvoltage on the one hand and the gain and thus the voltage at the outputof the multiplier M1 on the other hand.

The main advantage of this circuitry is that there is a smaller dynamicrange at the output of the multiplier Ml than the usual multipliers withwhich the signals to be multiplied are directly supplied to themultiplier. ln

contrast to the prior art the one input of the multiplier Ml receives asignal which is already divided by the value of the output signal of theaverager. Thus, the additional provision of a multiplier brings alongthe ad- 'vantage that after the transient time of the circuitry therange of voltage levels at the input of the first multiplier M1 issubstantially decreased; This will result in a considerable improvementof the square characteristic of the RMS measuring apparatus.

In FIG. 3a a typical input signal and its envelope are illustrated. Theillustrated input signal can be representative for a voltage curvederived from the noise pressure levels generated by the stroke of atypewriter.

The dotted lines of FIG. 3b show the response of prior art RMS meters.The leading edge as well as the trailing edge of the response has anexponential waveform and the discharge time constant or the trailingedge time constant is greater than the constant of the leading edge. Itcan be taken from the figure that with the special illustrated inputsignal the second typewriter stroke is completely covered by theexponential function of the preceding stroke so that the RMS value ofthe second stroke is not measured.

However, the solid curve of FIG. 3!; illustrates that the trailing edgeof the response function of the measuring apparatus according to theinvention decreases linearly to zero in a time interval oftwo timeconstants, where the response curve due to the second typewriter strokeis no longer obscured. Thereby a decisive improvement of the dynamicproperties of RMS meters is achieved which will especially work out withshort signals having high amplitudes.

We claim:

1. Apparatus for the continuous measurement of approximate RMS values ofan input signal, the apparatus comprising signal squaring means, anaverager connected to said squaring means and having an energystorageelement that provides an exponential transient characteristic, negativefeedback means connecting said averager to the signal squaring means forproviding signal division in proportion to the value of the outputsignal of the averager, source means of constant current connected tosaid averager for selectively discharging the energy-storage elementthereof with a substantially constant current during the period that theoutput of the averager is greater than a value indicative of theinstantaneous input signal, said source means supplying saidsubstantially constant current with an amplitude that is proportional tothe output of the averager upon the start of the discharge action.

2. The apparatus as in claim 1 wherein the squaring means includes apair of multipliers each having a pair of inputs, means applying inputsignal to one input of one of the multipliers, an operational amplifierincluding a feedback circuit connected to apply the input signal to theother input of said one of the multipliers, said feedback circuit ofsaid operational amplifier including the output and one input of theother of said multiplimeans with a reference potential for activatingthe switching means for the period during which the output potential ofthe squaring means is greater than the reference potential and fordeactivating the switching means in response to the output of thesquaring means attaining a value smaller than said reference potential,said source means supplying discharging current through said resistor tosaid integrating amplifier in response to deactivation of said switchingmeans.

5. The apparatus as in claim 1 wherein said source means supplies saiddischarge current at a value substantially equal to the initial value ofdischarge current associated with an exponential discharge which wouldresult without a constant current discharge.

Patent No. 3,795,868 Dated March 5, 1.974

' Inventor(s) Wolfgang Ohme, Karl Grund, Jorg Winkler It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1; in the second equation, lines 39-40,

cancel "-T/ZH" and Substitute -t /2-r Signed and sealed this 9th dayofJuly 1974.

(SEAL) Attest: I

MCCOY M. GIBSON, JR. C. MARSHALL DANN v Attesting Officer 7 Commissionerof Patents po'wso USCOMM-DC wan-pea

1. Apparatus for the continuous measurement of approximate RMS values ofan input signal, the apparatus comprising signal squaring means, anaverager connected to said squaring means and having an energy-storageelement that provides an exponential transient characteristic, negativefeedback means connecting said averager to the signal squaring means forproviding signal division in proportion to the value of the outputsignal of the averager, source means of constant current connected tosaid averager for selectively discharging the energy-storage elementthereof with a substantially constant current during the period that theoutput of the averager is greater than a value indicative of theinstantaneous input signal, said source means supplying saidsubstantially constant current with an amplitude that is proportional tothe output of the averager upon the start of the discharge action. 2.The apparatus as in claim 1 wherein the squaring means includes a pairof multipliers each having a pair of inputs, means applying input signalto one input of one of the multipliers, an operational amplifierincluding a feedback circuit connected to apply the input signal to theother input of said one of the multipliers, said feedback circuit ofsaid operational amplifier including the output and one input of theother of said multipliers, the other input thereof being connected tothe output of the averager.
 3. The apparatus according to claim 1wherein the averager comprises an operational amplifier includingcapacitive feedback for operating as an integrating amplifier, and saidsource means is connected through a resistor for discharging theintegrating amplifier.
 4. The apparatus according to claim 3 whereinsaid source means includes signal-controlled switching means connectedbetween the output of the integrating amplifier and said resistor, and asignal level comparator connected to compare the output of the squaringmeans with a reference potential for activating the switching means forthe period during which the output potential of the squaring means isgreater than the reference potential and for deactivating the switchingmeans in response to the output of the squaring means attaining A valuesmaller than said reference potential, said source means supplyingdischarging current through said resistor to said integrating amplifierin response to deactivation of said switching means.
 5. The apparatus asin claim 1 wherein said source means supplies said discharge current ata value substantially equal to the initial value of discharge currentassociated with an exponential discharge which would result without aconstant current discharge.